Flexxaire Manufacturing Inc. of Edmonton, Canada, manufactures a hydraulically controlled fan, and a pneumatically controlled fan. The pneumatic fan uses a single acting spring return piston, and the hydraulic fan uses a double acting piston. Examples of these fans are shown in FIGS. 1 and 2 of U.S. Pat. No. 7,229,250 issued Jun. 12, 2007. The pneumatic fan (AX) has a compact construction, which suits many applications where space is limited on the equipment on which it is used. However, the hydraulic fan (FX) also has advantages due to the use of hydraulic fluid as the drive force for the pitch change mechanism. A combination of compact fan with hydraulic drive would be desirable, but the design poses engineering challenges.